Open and short tests are required for manufacturing high density printed circuit boards and ceramic substrates. As disclosed in co-pending application Ser. No. 843,672 filed Feb. 20, 1992 by S. W. Ching et al., assigned to the same assignee as the instant invention and which disclosure is incorporated by reference to the instant application, circuit boards and ceramic substrates (to be included hereinafter as part of circuit boards) usually include metal interconnection networks (nets), power planes, dielectric materials, and at least one reference plane. Each of such circuit boards can in fact comprise multi-layers each having fabricated thereon different nets. Each net may also be distributed across the multi-layers within a circuit board.
In view of the high density in which electronic components are packaged onto each circuit board, the integrity of each circuit board, i.e. the integrity of the different metal interconnections that effect each of the nets in the circuit board, is of paramount import. Thus, defects such as an "open circuit" condition whereby higher than expected resistance appears between certain sections of a network, or a "short circuit" condition whereby two separate nets, which theoretically should have infinite resistance therebetween, in fact appear to be shorted together, or have an unacceptable internet (leakage) resistance, are to be detected and avoided.
To perform the open and short tests, prior art systems and methods use full cluster probes (gang probes or bed-of-nails probes) or a number of moving probes (serial testing). The full cluster probe method detects opens by measuring the resistance between the terminals of the nets, and shorts by measuring the resistance between the being tested net and the rest of the nets. The serial testing method detects opens by measuring the resistance of the net, and shorts by measuring the capacitance between the net and a reference plane. Measured excess capacitance indicates that a short exists. The serial testing method uses DC resistance meters for open detection and impedance meters such as HP 4284A LCR meter for capacitance measurements.
An exemplary prior art method in which two moving probes are used to perform tests on nets is disclosed in Burr et al. U.S. Pat. No. 4,565,966. As disclosed, Burr performs a series of one point measurements of the capacitance of a network relative to a reference plane. To test the continuity of each net, a resistance measurement is effected between two probes each placed at an end point of the net. To measure excessive internal capacitance between nets, Burr uses conventional sinusoidal AC signal generating devices. See column 5, lines 65-69 of Burr.
Another exemplary method of detecting shorts based on using AC signals is disclosed in the above noted copending '672 application. There, an AC phase sensitive method for effecting capacitive determination is disclosed. To measure the continuity of each net, resistance measurement is used.
There are several drawbacks to the above noted prior art systems and methods. For one, the throughput for short detection based on an AC capacitance measurement method is slow. This is due to the fact that the use of an AC method to detect shorts via capacitance measurement requires that .omega.R.sub.i C.sub.n (a quantative relationship) be less than or equal to unity. (.omega. is the angular frequency, R.sub.i the leakage resistance between the nets, and C.sub.n the capacitance of the net to which the being tested net is shorted.) For high leakage resistance, the unity criterion requires that low frequencies be used. So, too, the AC phase sensitive detection method requires that the time constant be set to at least one period of the AC signal. And since the rise time of the AC signal, or more precisely the response thereto, is an exponential function, an input signal having a long duration is required in order to produce a substantially accurate final value.
To isolate defects, the prior art methods (not including the copending '672 invention) would match the capacitance value of the nets that are shorted with nets that have similar capacitance values. Such defect isolation methods fail to take into account the leakage resistance that occurs between nets.
As was noted previously, the threshold of the prior art method (not counting copending '672 invention) is set by the relationship .omega.R.sub.i C.sub.n &lt;1. Since frequency f (.omega.=2.pi.f) is the product of R.sub.i C.sub.n, R.sub.i being the definition of short and C.sub.n being the highest possible value of net capacitance in the product, overkills for low capacitance nets result. For example, if the frequency f were to depend from a threshold definition based on a short of a 100 kohms multiplied with a 50 pF net, the prior art method would interpret a net of 1 pF (net to reference plane) and 5 megohms leakage resistance to be a short because its R.sub.i C.sub.n value is the same as the threshold value set for the definition of short R.sub.i and the maximum capacitance C.sub.n.
If the relationship .omega.R.sub.i C.sub.n &lt;1 is not satisfied, either it would be an escape or the measured capacitance is not the combined capacitance of the net under test and the net to which it is shorted. In the latter case, there would be a problem in isolating the short.